Although most asphalts are used for paving, some asphalts are used for other applications such as roofing. Not all available asphalts are naturally suitable for roofing applications. Asphalts for roofing are air-blown to raise the softening point of the asphalt and to meet other specifications. One way to utilize more asphalt feedstocks for roofing is to add a ferric chloride or ferrous chloride catalyst to the asphalt before the air-blowing process. The ferric chloride or ferrous chloride improves asphalt properties such as penetration at a targeted softening point and accelerates the air-blowing process to reduce processing time.
Unfortunately, hydrogen chloride emissions are generated when an asphalt modified with ferric chloride or ferrous chloride is air-blown. When the asphalt is modified with ferric chloride, the reduction of ferric chloride to ferrous chloride during the air-blowing process generates hydrogen chloride emissions. When ferric chloride is added as an aqueous solution to the asphalt, hydrogen chloride emissions are also generated from free hydrochloric acid present in the aqueous solution. More emphasis is being put on regulating the levels of hydrogen chloride emissions to reduce air pollution, and this trend will increase with time. If the regulated emission levels are not achieved, the asphalt production with ferric chloride or ferrous chloride will be restricted. Current methods for reducing hydrogen chloride emissions from an asphalt air-blowing process do not exist.
The patent literature does not suggest a suitable solution to the problem of hydrogen chloride emissions. U.S. Pat. No. 2,506,283 to Smith et al. discloses the use of ferric chloride as a catalyst during an asphalt air-blowing process, and the addition of calcium oxide or calcium hydroxide to the air-blown asphalt in a separate mixing operation after the air-blowing process is completed.
The purpose of the Smith et al. patent is to prevent the formation of scum on the surface of the asphalt, not to reduce hydrogen chloride emissions. There is no suggestion to add a chemical modifier to the knockout tank after the air-blowing but before the fume stream from the air-blowing process is emitted to the atmosphere.
U.S. Pat. No. 5,611,910 to Marzari et al. discloses a method for reducing SO.sub.X emissions from an asphalt air-blowing process by adding an emission reducing additive to the asphalt prior to air-blowing or early in the process. The additive comprises: (a) at least one compound selected from metal hydroxides, metal oxides, metal carbonates and metal bicarbonates, where the metal is selected from calcium, sodium, potassium and magnesium; and (b) at least one compound selected from metal hydroxides, metal oxides, metal carbonates and metal bicarbonates, where the metal is selected from zinc, copper and aluminum.
The Marzari et al. patent does not disclose the use of ferric chloride or ferrous chloride, or the resulting problem of hydrogen chloride emissions. There is no discussion of a method for reducing hydrogen chloride emissions, and in particular, there is no suggestion to add a chemical modifier to the liquid seal of a knockout tank to reduce hydrogen chloride emissions. Accordingly, it would be desirable to provide a method for reducing hydrogen chloride emissions from air-blowing an asphalt modified with ferric chloride.